A telescopic gun sight, commonly called a rifle scope, is a device used to provide an accurate point of aim for firearms such as rifles, handguns and shotguns. It is used with other types of weapons such as bows and crossbows as well. A telescopic sight can dramatically improve the functionality of a firearm by providing the shooter with a simple yet highly accurate means for aiming at distant targets.
A simplified schematic diagram of a modern telescopic sight is shown in FIG. 1. With reference to this figure, a telescopic sight uses an objective lens or lens group 1 to form a real image of the target at the objective focal plane 4. The image formed by the objective is upside-down and needs to be erected. The pair of convex lenses 3a and 3b erect the image formed by the objective and relay it to the eyepiece focal plane 5. Finally, an ocular (eyepiece lens) 2 is used to convert this erected real image into a virtual image at infinity for viewing by the shooter.
In telescopic sights, the “point of aim” is usually designated by a reticle or cross hairs. Reticles are most commonly represented as intersecting lines in a “+” shape though many variations exist, including dots, posts, circles, scales, chevrons, or a combination of each. With reference to FIG. 1, the reticle is placed either at the objective focal plane 4 or the eyepiece focal plane 5. These two planes are also referred to as the First Focal Plane (FFP) and the Second Focal Plane (SFP), respectively. In either case, the reticle's shape is superimposed on the target image providing a precise indication of the point of aim.
Modern telescopic sights are equipped with at least two control knobs for elevation (up-down) and windage (left-right) adjustments. These knobs allow for precise vertical and horizontal movement of the reticle so that the sight's point of aim can be aligned with the firearm's “point of impact”.
It is very desirable to be able to keep the reticle at the center of field of view while its position is adjusted inside the telescopic sight. To achieve this, almost all rifle scopes manufactured today use a tilting inner tube mechanism similar to the one disclosed in U.S. Pat. No. 3,161,716 issued Dec. 15, 1964 to D. J. Burris and J. L. Maulbetsch. A simplified version of this mechanism is shown in FIG. 1.
With reference to FIG. 1, the mechanism to keep the reticle at the center of field of view includes an inner tube 6 that contains the reticle 20 and the image erection lenses 3a and 3b. The back end of the inner tube 6 is connected to the main casing 10 by a special joint 7. The front end of the inner tube 6 is connected to an adjustment knob 8 and is supported by a leaf spring 9. Adjusting the knob 8 tilts the inner tube 6 up and down. This, in turn, moves the reticle 20 which is mounted inside the inner tube with respect to the target image formed by the objective and adjusts the point of aim. Since the reticle and the erecting lenses 3a and 3b are tilted together, the reticle image always appears at the center of field of view. (Note: For simplicity and clarity, only up-down adjustment is shown in FIG. 1. Adjustment for left-right is achieved similarly by tilting the inner tube side ways.)
The tilting inner tube mechanism described above is used by almost all rifle scope manufacturers today. However, it has several drawbacks:
A first drawback is that the possible amount of reticle adjustment is limited to the clearance between the inner tube and the scope's main tube. Telescopic sights must use a standard main tube diameter (one inch in the US and 30 millimeters in Europe). Furthermore, optical considerations make using a small diameter inner tube impractical. Therefore, the titling tube mechanism is naturally limited in terms of the amount of elevation and windage adjustments possible.
A second drawback is that due to the circular shape of the inner tube and the scope's main tube, vertical and horizontal movements of the inner tube interfere with each other near the ends of the adjustment range. For example, if the inner tube is tilted upwards near the end of its adjustment range, then it can not move much in the left and right directions. This phenomenon is shown graphically in FIG. 2. In this figure, the circular region 22 indicates the maximum amount of adjustment possible in each direction while the smaller rectangular region 21 indicates the area in which elevation and windage can be adjusted independently without one limiting the other.
A third drawback of the tilting inner tube solution is that it requires a precise and expensive joint mechanism (often implemented as a ball-and-socket joint) at its rear end. This joint must allow the inner tube to tilt up-down and left-right with very high degree of precision and also withstand the shock and vibrations caused by the weapon's recoil force.
A forth drawback is that the adjustment knob and the return spring used to tilt the inner tube must be of very high quality and without hysteresis. These conditions are very difficult to satisfy in practice and even the highest quality rifle scopes manufactured today are susceptible to shift in their point of aim if their adjustment knobs are turned and then reverted back to the original position.
The reader is referred to U.S. Pat. No. 5,463,495 issued Oct. 31, 1995 to E. Murg and U.S. Pat. No. 5,615,487 issued Apr. 1, 1997 to S. Tomita for more information on the difficulties associated with the design of the tilting inner tube mechanism.
This patent teaches an optical method for adjusting the point of aim in a telescopic sight wherein the reticle is fixed with respect to the sight's optical axis. This method eliminates the issues associated with the tilting-inner-tube solution used in the prior art.